To produce selective blocks of the Cerebrospinal Fluid (CSF) pathways and study the resulting changes in CSF absorption. A silastic mixture that has given good results in our laboratory will be used to produce the blocks (Ahmadi, et al. 1979). Blocks will be placed in dogs as follows: 1. Basal cisterns, 2. the arachnoid granulations projecting into the sagittal venous sinuses, 3. the olfactory bulbs, 4. the upper spinal subarachnoid space (C1-C2), 5 fourth ventricle. While blocks in the basal cisterns and those in the fourth ventricle induce hydrocephalus, it is uncertain whether specific blocks in the other locations will induce ventricular enlargement. It appears quite probable, however, that selective blocks of known CSF absorptive pathways will give dynamic and morphologic indications of the contributions of such pathways under the experimental conditions. The silastic produces blocks devoid of arachnoiditis. Particular importance will be given to the distribution of paramagnetic enhancers and measurements of aqueductal flow. These studies will include observations of three sets of markers: 1) Radiologic water-soluble nonionic contrast media (iohexol and metrizamide); 2) labeled molecules (RISA and C14 sucrose) (to determine, by perfusion techniques, rates of CSF production and absorption); and 3) morphologic markers (for light and electron microscope studies): horseradish peroxidase (50,000 daltons). and lanthanum chloride (400 daltons). Radiologic and imaging studies will include plain radiolgraphy, tomography, CT and MRI during critical periods of the development of pathological changes in every case. It is expected that the correlation of imaging physiologic and morphologic findings will help the clinician to determine the physio-pathology more accurately and diagnose patients under treatment. To produce experimental methods to rapidly wash radiopaque media (iohexol and metrizamide) from the subarachnoid space after cranial and lumber myelography to lower the risk of their remaining in the subarachnoid space for a long period of time.